Low pressure mercury vapour electric discharge lamps



April 7, 1959 c. HlGGlNS ET AL 2,881,345

LOW PRESSURE MERCURY VAPOUR ELECTRIC DISCHARGE LAMPS Filed Feb. 2, 1954 INVENTORS n-r-mPue United States Patent LOW PRESSURE MERCURY VAPOUR ELECTRIC DISCHARGE LAMPS Application February 2, 1954, Serial No. 407,661

Claims priority, application Great Britain February 3, 1953 Claims. (Cl. 313220) This invention relates to low pressure mercury vapour electric discharge tubes for emitting light, of the type having an elongated tubular glass envelope provided with an electrode at each end and containing a filling of rare gas and mercury such that, in normal operation of the tube, the space between the electrodes is occupied mainly by the positive columnof a discharge through the mercury vapour. The purpose of the rare gas, which usually has a pressure of a few millimetres rise of mercury column, is to facilitate starting of the discharge between the electrodes when the tube is energised; the mercury vapour pressure in normal operation is a fraction of a millimetre rise of mercury column. Such tubes are at present widely used for advertising and display purposes and as lamps for electric lighting, and are often provided with a coating of fluorescent material for converting invisible ultra-violet radiation from the mercury discharge intouseful visible radiation.

When a tube of the type with which the invention is concerned is operated in conditions of low ambient temperature, the regions around and adjacent to the electrodes are liable to darken after the tube has been running for a time. This change is due to cooling of the central portion of the tube envelope by the cold surrounding atmosphere, which results in withdrawal of mercury from the hotter regions around the electrodes into the colder central region of the envelope, this withdrawal of mercury taking place gradually until no mercury remains in the end parts of the envelope, and consequently light is emitted almost entirely from the central region of the envelope, with very little or no light from the regions around and adjacent to the electrodes. This phenomenon may occur, for example, with low pressure mercury tubes employed out-of-doors for advertising or display purposes, and may also occur, for example, with tubes used for lighting purposes in artificially cooled storage rooms. The effect may be experienced with tubes having heatable thermionic cathodes as well as with tubes having cold cathodes.

Means for mitigating this effect have been proposed, and the object of the present invention is to provide a tube of the type specified incorporating improved means for preventing or retarding the advent of the'darke'ning of the end regions, resulting from the said efiect.

According to the invention, in an electric discharge tube of the type specified each end part of the envelope is partitioned ofl from the rest of the envelope by means of a partition member which extends across the envelope behind the electrode at that end and fits closely against the inner wall of the envelope, each said end part contains liquid mercury, and each said partition member is provided with one or moreorifices for permitting the passage of mercury vapour from said end part into'the main body of the envelope, at least one of said orifices being such that, if not prevented, liquid mercury would leak therethrough from said end part, and each such orifice being fitted with an eyelet having a tubular mem- 2,881,345? Patented Apr. 7, 1959 "ice '2 ber projecting into the end part for a sufiicient distance to form a trap which prevents the leakage of liquid mercury from the said end part through the orifice in any position of tilting of the envelope, whilst still permitting the ready passage of mercury vapour through the orifice.

The term closely in the foregoing statement implies that each partition member fits against the wall of the envelope so tightly that liquid mercury from the end part of the envelope cannot leak round the periphery of the partition member. The phrase behind the electrode means, of course, situated between the electrode and the end wall of the envelope.

Each partition member is preferably constituted by a disc or washer carried by the electrode or the electrode support means so as to be inserted into the tube envelope simultaneously with the electrode in the manufacture of the tube, the disc or washer being arranged to abut closely around its periphery to the inner wall of the envelope. Preferably the partition member is spaced away from the electrode, in order to reduce the rate of transference of heat from the electrode to the end part of the envelope and thus prevent too rapid volatilisation of the mercury. Thus a disc or washer forming the partition member may be carried by leadingin wires which also support the electrode and may be located on, or close to, a pinch through which these wires are sealed. Further, in order to minimize heat transference as aforesaid, the partition member is preferably formed of a material of low conductivity.

The partition member may, if desired, be constructed of material which is permeable to mercury vapour, such as woven glass wool. This, however, is not necessary,

.since the orifice ororifices provided in the partition member allow the passage of mercury vapour into the main body of the envelope at a sufliciently rapid rate; accordingly the partition member may conveniently be constructed of non-permeable material such as mica. Preferably, during the manufacture of the discharge tube, the envelope is shaped to bring it into close contact with the partition member around the whole of its periphery; thus if the partition member is a disc composed of a material which seals to glass, the whole of the peripheral surface of the disc may be sealed to the glass wall of the envelope; or if the disc is composed of a material, such as mica, which does not seal to glass, the glass may be softened and rolled on to the periphery of the disc when the latter is in its required position in the envelope, so as to ensure complete contact of the peripheral surface of the disc with the glass.

Eyelets having tubular extensions similar to those fitted to the orifices provided for the passage of mercury vapour may, if desired or necessary, also be fitted to the orifices through which the leading-in wires are arranged to pass from the end of the envelope to the cathode, and such eyelets may in some cases be used to secure the par tition member to the leading-in wires. The provision of eyelets in this case is, however, not necessary if the wires fit into these orifices sufiiciently closely to prevent leakage of liquid mercury therethrough.

The mercury contained in the reservoirs at each end of a discharge tube in accordance with the invention may constitute the whole of the mercury filling of the tube,

vbut in some cases additional mercury may be initially istrapped in the end reservoirs, larger amounts of mercury can be employed than have been considered desirable hitherto, when the liquid mercury filling has been free to move about in the envelope, during transportation and installing of the tube, with the consequent danger of damaging the fluorescent coating, if present.

An additional advantage arising from the provision of one or more relatively large eyeletted orifices in the partition members of a tube in accordance with the invention is that this arrangement facilitates the evacuation and degassing of the envelope by pumping during the manufacture of the tube. We have found that, to obtain the most efficient operation of the tube, the end parts of the envelope, which serve as mercury reservoirs, should be substantially completely exhausted during the pumping' process, and the provision of orifices fitted with eyelets as aforesaid makes this possible without the risk of liquid mercury subsequently escaping from the reservoirs through the orifices provided.

One specific discharge tube in accordance with the in- I vention is shown in Fig. l of the accompanying drawing while Fig. 2 shows an element of said discharge tube, and will now be described by way of example together with the method of manufacturing the tube.

Figure l of the drawing shows a perspective view of the tube with the central part broken away, the tube in this case being a cold cathode lamp of the kind used for exterior decorative lighting. The envelope of the lamp is 9 feet long, and consists of a central portion 1 which is 20 millimetres in diameter, the two end portions 2 comprising the cathode chambers and mercury reservoirs, these latter portions being 2.7 millimetres in diameter and 4 inches long. The lamp is designed to operate at a current of 120 milliamps, the temperature of the cathode during operation being about 200 C. The lamp is provided with an internal coating of fluorescent material, which has not been shown in the drawmg.

Each of the end portions 2 of the lamp contains a cylindrical cathode 3, to the inner end of which is attached a ceramic ring 4, which serves to protect the edge of the cathode from the discharge. A space 5 atthe rear of each cathode, which space contains a globule of mercury 6, is partitioned off by a mica washer 7, approximately 10 mils in thickness, which abuts against an indentation 8 in the envelope wall, the indentation serving to hold the washer in position and to prevent any of the mercury globule from leaking past the washer into the main body of the lamp envelope. A small orifice 9 is provided in the washer 7, and a nickel eyelet with a tubular extension 10, Ms inch in length and "A inch internal diameter, is fitted to the orifice 9 so as to extend into the end space 5, as shown in Figure 2, which is an enlarged sectional view of the washer 7. The leading-in wires 11, which are sealed into the envelope through the glass pinch 12, pass through the washer 7 through closely fitting orifices 13 (Figure 2), and a T-shaped wire assembly 14 is placed in contact with the washer on the cathode side thereof and is welded to one of the leading-in wires, to brace the washer and ensure that it is held firmly in position against the pinch 12.

Each globule of mercury weighs about 1 to 3 grams, and the two globules of mercury contained in the two end parts of the envelope fo'rmthe entire mercury filling of the lamp; the lamp also contains a mixture of argon and neon at a pressure of to 6 millimetres rise of mercury column.

In the manufacture of the lamp shown .in Figure 1 of the drawing, for each cathode assembly the leadingin wires 11 are sealed in through the pinch 12, and the mica washer 7, T-assembly 14,- and cathode 3 are assembled and secured in position, the ends of the wires 11 being bent and welded to the cathode, Each. cathode assembly is then sealed into one end of the envelope and the indentation 8 is then formed by softening the glass by allowing a flame to impinge upon it around a zone adjacent to the periphery of the washer 7, and pressing the softened glass on to the periphery of the washer by means of a roller. The envelope is then evacuated from one end, the eyeletted traps provided in accordance with the invention permitting the evacuation of the whole of the envelope in this way; the rare gas filling is passed into the envelope, and the mercury is introduced into each of the reservoir chambers 5 through a stem 15 provided in the pinch at that end of the envelope, one of which stems 15 serves also as the pumping stern, and both of which stems 15 are sealed off after the evacuation of the envelope and the introduction of the gas filling and the mercury. Finally the amp is capped in the usual manner.

The lamp shown in the drawing and described above by way of example may be modified by providing one or more additional eyeletted orifices, 9, with tubular extensions 10 of the eyelets, in the mica washer 7. A further possible modification consists in the provision of similar eyelets for the orifices 13 through which the leading-in wires 11 pass. Neither of these modifications isshown in the drawing.

We claim:

1. An electric discharge tube of the type having an elongated tubular glass envelope provided with an electrode at each end and containing a filling of rare gas and mercury such that, in normal operation of the tube, the space between the electrodes is occupied mainly by the positive column of a discharge through the mercury vapour and the envelope being so long that the middle part between the electrodes runs at a lower temperature than the two end parts behind the electrodes, wherein each end part of the envelope is partitioned off from the rest of the envelope by means of a partition member which extends across the envelope behind the electrode at that end, being spaced therefrom so as to be out of conductive thermal contact therewith, and fits closely against the inner wall of the envelope, a pumping stem at said end terminating short of the partition, each said end part contains a replacement supply of liquid mercury, and each said partition member is provided with one or more orifices for permitting the passage of mercury vapour from said end part into the main body of the envelope, at least one of said orifices being of such cross-section, that if not prevented, liquid mercury would leak therethrough from said end part, and each such orifice being fitted with an eyelet having a tubular member projecting into the end part for a sufficient distance to form a trap which prevents the leakage of liquid mercury from the said end part through the orifice in any position of tilting of the envelope, whilst still permitting the ready passage of mercury vapour through the orifice.

2. An electric discharge tube according to claim 1 wherein each partition member is constituted by a disc carried by the electrode support means and arranged to abut closely around its periphery against the inner wall of the envelope.

3. An electric discharge tube according to claim 1, wherein each partition member is composed of mica.

4. An electric discharge'tube according to claim 2, wherein each partition member abuts around the whole of its periphery against the interior surface of an annular indentation formed in the tube envelope.

5. An electric discharge tube of the type having an elongated tubular glass envelope provided with an electrode at each end and containinga filling of rare gas and mercury such that, in normal operation of the tube, the space between the electrodes is occupied mainly by the positive column of a discharge through the mercury vapour and the temperature distribution along the envelope has a minimum at a central region between the two electrodes which is less than the temperature at the two ends of the envelope, wherein each end part of the envelope is '5 partitioned oil from the rest of the envelope by means of a disc which extends across the envelope behind the electrode at that end and fits closely against the inner wall of the envelope, each said end part contains a replacement supply of liquid mercury, each said disc is provided with at least one orifice for permitting the passage of mercury vapour from said end part into the main body of the envelope on heating of said end part by heat from the discharge in operation of the lamp, at least one of said orifices being of such'cross-section that, if not prevented, liquid mercury would leak therethrough from said end part, each such orifice is fitted with an eyelet having a tubular member projecting into the end part for a sufiicient distance to form a trap which prevents the leakage of liquid mercury from the said end part through the orifice in any position of tilting of the envelope, whilst still permitting the ready passage of mercury vapour through the orifice, and each disc and electrode assembly is carried on wires sealed through a glass pinch closing the corresponding end of the envelope, which pinch is provided with a sealed pumping stem terminating short of the disc.

References Cited in the file of this patent UNITED STATES PATENTS 1,820,392 Hotchner Aug. 25, 1931 1,930,090 Gaides Oct. 10, 1933 1,961,897 Bol June 5, 1934 1,963,008 Weeks June 12, 1934 2,009,221 Bruiines et al July 23, 1935 2,056,621 Roberts Oct. 6, 1936 2,123,957 Orth July 19, 1938 2,182,780 Alterthum et al Dec. 12, 1939 2,433,404 Smith Dec. 30, 1947 2,443,632 Miller June 22, 1948 2,443,633 Miller June 22, 1948 

